Earthmoving and construction work machines often employ a hydraulic steering system that controls steering functions and operation. Some conventional hydraulic steering systems incorporate a hydro-mechanical valve that regulates fluid flow to steering actuators on the work machines. A direct mechanical connection with a steering wheel opens the hydro-mechanical valve in proportion to any steering wheel rotation. Other conventional hydraulic steering systems incorporate a hand-metering unit associated with the steering wheel to regulate fluid flow to the steering actuators.
More recently, some work machines employ electro-hydraulic type steering systems. These steering systems incorporate sensors that detect the rotation or movement of a steering device, such as the steering wheel. A controller may communicate with the sensor to monitor the movement of the steering wheel. Based upon the monitored movement, the controller may generate a control signal that electronically controls a hydraulic valve to allow a responsive fluid flow to the steering actuators. Such systems are less complex, less expensive, and more compact than previous hydro-mechanical steering systems.
Because steering is an important function on a work machine, some work machines include a primary hydraulic steering system and an auxiliary or secondary hydraulic steering system. If the primary hydraulic steering system is disabled or is not operating, as might occur during a valve failure, an engine failure, a pump failure, or other failure, the auxiliary or secondary hydraulic steering system may direct fluid to provide continuous steering control to the operator. In the event that the primary steering system is not operating, the work machine may be configured to automatically switch from the primary steering system to the auxiliary or secondary steering system.
One hydraulic system having a primary and a backup steering system is disclosed in U.S. Pat. No. 6,039,133. The '133 patent discloses a steering control system employing first and second steering control valves. The first steering control valve includes a first pair of actuating chambers and the second steering control valve includes a second pair of actuating chambers. The first steering control valve is controlled by a first pair of electro-hydraulic pilot valves through the first pair of actuating chambers. The second steering control valve is controlled by a second pair of electro-hydraulic pilot valves through the second pair of actuating chambers. If the first steering control valve fails, the second steering control valve may provide pilot fluid to the second pair of actuating chambers, through mechanical pilot valves.
The system disclosed in the '133 patent may be more complex than desirable or necessary. For example, the system requires the second steering control valve to have two pairs of actuating chambers in order to provide control. Because the system is complex, it may require more maintenance than is desirable. Furthermore, because the system is complex, it may be overly expensive.
The electro-hydraulic system disclosed and described herein may overcome one or more of the problems in the prior art.